US20100208232A1 - Tracking-type laser interferometer - Google Patents

Abstract

A tracking-type laser interferometer in which a pattern emission control unit controls a changing mechanism such that light is emitted along a predetermined pattern when judged by a first judgment unit that at least one of received-light amounts at first and second light reception units is not greater than a first threshold value. A tracking control unit causes the changing mechanism to keep track of a retro reflector when judged by a second judgment unit that both of the received-light amounts at the first and second light reception units are greater than second threshold values during a time period in which the pattern emission control unit controls the changing mechanism for the emission of light along the pattern. The interferometer emits light along the pattern to search for the retro reflector upon losing sight thereof. Upon detection, the interferometer can keep track of the reflector again and resume measurement.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The invention relates to a tracking-type laser interferometer.
• 2. Description of the Related Art
• A tracking-type laser interferometer is used as an apparatus for measuring a distance to a movable body. An example of a tracking-type laser interferometer of related art is disclosed in Japanese Unexamined Patent Application Publication No. 2008-128899. The tracking-type laser interferometer disclosed in the above patent document splits a beam of laser light into a beam of measurement light and a beam of reference light. The interferometer emits the measurement light toward a retro reflector that is attached to the movable body. The interferometer performs control processing such that the shift amount of return light propagating back from the retro reflector should fall within a predetermined range, thereby keeping track of the retro reflector. The reference light is reflected at the reference plane. The reference light reflected at the reference plane and the return light propagating back from the retro reflector turn into interference light. The interferometer utilizes the interference light to measure a distance therefrom to the retro reflector (movable body).
• Backward light that is supposed to propagate back from a retro reflector will not actually return to an interferometer when measurement light is not properly directed at the retro reflector or when there is an obstacle between the interferometer and the retro reflector. In such cases, the tracking-type laser interferometer of related art loses sight of the retro reflector and thus continues emitting measurement light blindly in the lost direction. For this reason, the tracking-type laser interferometer of related art cannot resume the measurement of a moveable object when the interferometer loses sight of the retro reflector, which is a problem that remains to be solved.
SUMMARY OF THE INVENTION
• An advantage of some aspects of the invention is to provide a tracking-type laser interferometer that is capable of resuming measurement even when the interferometer loses sight of a retro reflector.
• A tracking-type laser interferometer according to a first aspect of the invention has the following features. The tracking-type laser interferometer includes a light source, a retro reflector, a reference plane, a first light receiving section, a second light receiving section, a changing mechanism, a changing mechanism controlling section, and a distance calculating section. The retro reflector is attached to a movement member and reflects light propagating from the light source. The reference plane reflects light coming from the light source. The first light receiving section receives interference light turned from return light and reference light. The return light propagates back from the retro reflector. The reference light is reflected at the reference plane. The first light receiving section outputs a received-light signal dependent on received-light amount and a change in a distance to the retro reflector upon receiving the interference light. The second light receiving section receives the return light to output a received-light signal dependent on received-light amount and shift amount of the return light. The changing mechanism changes a direction of emission of the light propagating from the light source. The changing mechanism controlling section controls the changing mechanism on the basis of the received-light signal outputted from the second light receiving section such that the shift amount should fall within a predetermined range to cause the changing mechanism to keep track of the retro reflector. The distance calculating section calculates the distance from a predetermined reference point to the retro reflector by means of the received-light signal outputted from the first light receiving section. The changing mechanism controlling section includes a first judging section, a second judging section, a pattern emission controlling section, and a tracking controlling section. The first judging section judges whether at least one of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section is not greater than (i.e., is less than or equal to) a predetermined first threshold value, which is set individually for each of the first and second light receiving sections. The pattern emission controlling section controls the changing mechanism such that the light propagating from the light source should be emitted along a predetermined pattern in a case where it has been judged by the first judging section that at least one of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section is not greater than the predetermined first threshold value. The second judging section judges whether both of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section are greater than, or not less than, predetermined second threshold values, which are set respectively for the first and second light receiving sections. The tracking controlling section controls the changing mechanism such that the shift amount should fall within the predetermined range to cause the changing mechanism to keep track of the retro reflector in a case where it has been judged by the second judging section that both of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section are greater than, or not less than, the predetermined second threshold values.
• In the configuration of a tracking-type laser interferometer according to the above aspect of the invention, the pattern emission controlling section controls the changing mechanism such that the light propagating from the light source should be emitted along a predetermined pattern in a case where it has been judged by the first judging section that at least one of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section is not greater than (i.e., is less than or equal to) the predetermined first threshold value. Then, the tracking controlling section causes the changing mechanism to keep track of the retro reflector in a case where it has been judged by the second judging section that both of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section are greater than, or not less than, the predetermined second threshold values during a time period in which the pattern emission controlling section controls the changing mechanism for the emission of light propagating from the light source along the predetermined pattern. A tracking-type laser interferometer according to the above aspect of the invention, which may be hereinafter simply referred to as interferometer, emits a beam of light along the predetermined pattern to search for the retro reflector upon losing sight of the retro reflector. Upon detecting the retro reflector, the interferometer can keep track of the retro reflector again. Therefore, it is possible to resume measurement even when the interferometer loses sight of the retro reflector.
• A tracking-type laser interferometer according to a second aspect of the invention has the following features. The tracking-type laser interferometer includes a light source, a retro reflector, a reference plane, a first light receiving section, a second light receiving section, a changing mechanism, a changing mechanism controlling section, and a distance calculating section. The retro reflector is attached to a movement member and reflects light propagating from the light source. The reference plane reflects light coming from the light source. The first light receiving section receives interference light turned from return light and reference light. The return light propagates back from the retro reflector. The reference light is reflected at the reference plane. The first light receiving section outputs a received-light signal dependent on received-light amount and a change in a distance to the retro reflector upon receiving the interference light. The second light receiving section receives the return light to output a received-light signal dependent on received-light amount and shift amount of the return light. The changing mechanism changes a direction of emission of the light propagating from the light source. The changing mechanism controlling section controls the changing mechanism based upon the received-light signal outputted from the second light receiving section such that the shift amount should fall within a predetermined range to cause the changing mechanism to keep track of the retro reflector. The distance calculating section calculates the distance from a predetermined reference point to the retro reflector by means of the received-light signal outputted from the first light receiving section. The changing mechanism controlling section includes a first judging section, a second judging section, a pattern emission controlling section, and a tracking controlling section. The first judging section judges, on the basis of either one of the received-light signals outputted respectively from the first and second light receiving sections only, whether the received-light amount at the one light receiving section is not greater than the predetermined first threshold value or not. The pattern emission controlling section controls the changing mechanism such that the light propagating from the light source should be emitted along a predetermined pattern in a case where it has been judged by the first judging section that the received-light amount at the one light receiving section is not greater than the predetermined first threshold value. The second judging section judges, on the basis of the one of the received-light signals outputted respectively from the first and second light receiving sections only, whether the received-light amount at the one light receiving section is greater than, or not less than, the predetermined second threshold value or not. The tracking controlling section controls the changing mechanism such that the shift amount should fall within the predetermined range to cause the changing mechanism to keep track of the retro reflector in a case where it has been judged by the second judging section that the received-light amount at the one light receiving section is greater than, or not less than, the predetermined second threshold value.
• Since an interferometer according to the second aspect of the invention has some features that are the same as those of an interferometer according to the first aspect of the invention, it produces the same or similar advantageous effects. Besides these advantages, an interferometer according to the second aspect of the invention has an advantage of a simpler configuration because it is judged whether the interferometer has now lost sight of the retro reflector and whether the interferometer has now found the retro reflector on the basis of either one of received-light signals outputted respectively from the first and second light receiving sections only.
• In a tracking-type laser interferometer according to the first aspect of the invention or the second aspect of the invention, it is preferable that the predetermined pattern should be a spiral pattern that starts at a given point and goes away from the given point outward while turning around the given point as the center of the spiral pattern on a plane; the given point should lie in a direction of emission of light propagating from the light source when it is judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value; the plane should pass through the given point; and the plane should be orthogonal to the direction of emission of the light propagating from the light source.
• The emission pattern of light that is used when the interferometer searches for the retro reflector is a spiral pattern whose center is the given point that lies in the direction of emission of light when the interferometer lost sight of the retro reflector. With such a preferred feature, it is possible to search for the retro reflector efficiently without a detection failure, or with a substantially reduced risk of a detection failure.
• In the above preferred tracking-type laser interferometer, it is further preferable that the given point should be set at a position away from the reference point by a certain distance to the retro reflector calculated by the distance calculating section when judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value; the predetermined pattern should be a spiral pattern that includes a plurality of curved successive turnings and has a constant separation distance between the inner turning and the outer turning of each two adjacent turnings; and the constant separation distance between the inner turning and the outer turning of each two adjacent turnings of the spiral should be set at a value that is not larger than the width of a light-receivable area of the light receiving section for which it has been judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value.
• The emission pattern of light that is used when the interferometer searches for the retro reflector is a spiral pattern that includes a plurality of curved successive turnings and has a constant separation distance between the inner turning and the outer turning of each two adjacent turnings. In addition, the constant separation distance between the inner turning and the outer turning of each two adjacent turnings of the spiral is set at a value that is not larger than the width of a light-receivable area of the light receiving section for which it has been judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value. With such a preferred feature, it is possible to substantially reduce the risk of a detection failure.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a diagram that schematically illustrates an example of the configuration of a tracking-type laser interferometer according to an exemplary embodiment of the invention;
• FIG. 2 is a diagram that schematically illustrates an example of the configuration of an optical system;
• FIG. 3 is a diagram that schematically illustrates an example of a shift of return light from measurement light, which occurs due to the movement of a retro reflector;
• FIG. 4 is a diagram that schematically illustrates an example of a given point that is set by a pattern generation unit according to an exemplary embodiment of the invention, and the like;
• FIG. 5 is a diagram that schematically illustrates an example of a spiral pattern that is generated by the pattern generation unit according to an exemplary embodiment of the invention;
• FIG. 6 is a flowchart that schematically illustrates an example of the flow of a method for measuring a distance to a movable body by means of the interferometer according to an exemplary embodiment of the invention; and
• FIG. 7 is a diagram that schematically illustrates a spiral pattern according to a variation example of an exemplary embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
• With reference to the accompanying drawings, an exemplary embodiment of the present invention will now be explained.FIG. 1 is a diagram that schematically illustrates an example of the configuration of a tracking-type laser interferometer1 according to the present embodiment of the invention. The tracking-type laser interferometer1 is hereinafter simply referred to as “interferometer1”. Theinterferometer1 keeps track of amovable body2 and measures a distance therefrom to themovable body2. Themovable body2 is mounted on or provided as a component of an industrial machine. For example, the industrial machine drives a moving mechanism to move themovable body2, thereby measuring a target object or performs machining processing on the target object. An example of the industrial machine is a three coordinate measuring machine. An example of themovable body2 is a slider of the three-dimensional measuring machine. A probe for measuring a target object is fixed to the slider.
• As illustrated inFIG. 1, theinterferometer1 is provided with aretro reflector11, ameasurement unit12, and acontrol unit3. Thecontrol unit3 controls the operation of themeasurement unit12. Theretro reflector11 is attached to themovable body2. Theretro reflector11 reflects a beam of incident light with the following reflection characteristics. The propagation direction of a beam of reflected light and the propagation direction of the beam of incident light are parallel to each other. In addition, the reflected light and the incident light are centrosymmetric, that is, symmetric with respect to the center of the retro reflector11 (i.e., point symmetry). Therefore, in a case where the incident light enters theretro reflector11 at a certain position off the center, the path of the reflected light is shifted from the path of the incident light.
• Themeasurement unit12 is provided with anoptical system4 and a changingmechanism121. The configuration of theoptical system4 is known as described in detail in, for example, Japanese Unexamined Patent Application Publication No. 2008-128899. Therefore, the configuration of theoptical system4 is briefly explained below.
• FIG. 2 is a diagram that schematically illustrates an example of the configuration of theoptical system4.FIG. 3 is a diagram that schematically illustrates an example of a shift of return light, which is light after reflection at theretro reflector11, from measurement light, which is emitted from theoptical system4 and enters theretro reflector11 as incident light. The illustrated shift occurs due to the movement of theretro reflector11. InFIG. 3, two-dot chain lines show the position of theretro reflector11 before movement and the position of themovable body2 before movement. As illustrated inFIG. 2, theoptical system4 includes a distance measurementoptical system41 and a trackingoptical system42. The distance measurementoptical system41 is used for measuring a distance to theretro reflector11. The trackingoptical system42 is used for keeping track of theretro reflector11.
• The distance measurementoptical system41 includes alaser light source411, asplitter412, a plane mirror, and a firstlight reception unit413. The plane mirror, which is not illustrated in the drawing, is an example of a reference plane according to an aspect of the invention. The firstlight reception unit413 is provided with a photo detector (PD). The trackingoptical system42 includes asplitter421 and a secondlight reception unit422. The secondlight reception unit422 is provided with a quadrisected (i.e., four-divided) photodiode (PD) or a two-dimensional position sensitive detector (PSD).
• Theoptical system4 that includes the above optical components operates as follows. Thesplitter412 splits a beam of laser light emitted from thelaser light source411 into a beam of reference light, which is not shown in the drawing, and a beam of measurement light. The plane mirror reflects the reference light. Thereafter, thesplitter412 reflects the reference light toward the firstlight reception unit413. On the other hand, the measurement light that has passed through thesplitter421 is emitted toward theretro reflector11. The measurement light is reflected at theretro reflector11 to turn into return light, which is backward light propagating back toward theoptical system4. The return light enters theoptical system4. The measurement light sometimes enters theretro reflector11 at a certain position off the center thereof because of the movement of the retro reflector11 (refer toFIG. 3). In such a case, the measurement light is reflected with an optical shift orthogonal to, or in relation to, the direction of the incidence of the measurement light. Therefore, the path of the return light is shifted from the path of the measurement light.
• Some part of the return light that enters theoptical system4 is reflected at thesplitter421. The secondlight reception unit422 receives the light reflected by thesplitter421. The return light enters at a certain position off the center of the light reception plane of the second light reception unit (e.g., quadrisected PD)422 depending on the amount of the shift. The light reception plane of the secondlight reception unit422 is sectioned in four blocks, that is, the upper left section, the upper right section, the lower left section, and the lower right section. The secondlight reception unit422 generates four received-light signals. The level of each of the four received-light signals depends on the amount of the return light that enters the corresponding one of the four sections of the light reception plane. The secondlight reception unit422 outputs the four received-light signals (which may be hereinafter collectively referred to as a second received-light signal) to thecontrol unit3. In other words, the secondlight reception unit422 outputs the second received-light signal dependent on the amount of the received light and the shift amount of the return light to thecontrol unit3.
• The other part of the return light passes through thesplitter421. After passing through thesplitter421, the other part of the return light and the reference light reflected at the plane mirror turn into interference light, which is received at the firstlight reception unit413. Upon receiving the interference light turned from the remaining part of the return light and the reference light, the firstlight reception unit413 outputs a first received-light signal, which is dependent on the amount of the received light and a change in a distance between theoptical system4 and theretro reflector11, to thecontrol unit3. Each of the first received-light signal and the second received-light signal may be referred to as a received-light signal.
• Referring back toFIG. 1, the changingmechanism121 includes a rotation mechanism that has two rotation axes that are orthogonal to each other. Specifically, the changingmechanism121 includes an azimuthal angle rotation sub-mechanism and an elevation angle rotation sub-mechanism. The azimuthal angle rotation sub-mechanism changes the angle of direction of measurement light. The elevation angle rotation sub-mechanism changes the angle of elevation of measurement light. The changingmechanism121 drives the azimuthal angle rotation sub-mechanism and the elevation angle rotation sub-mechanism to change the direction of emission of measurement light, that is, the angle of direction thereof and the angle of elevation thereof. A sensor is mounted on each rotation sub-mechanism. The sensor mounted on the azimuthal angle rotation sub-mechanism detects the rotation angle of the axis (i.e., rotation shaft) thereof as the angle of direction of measurement light and outputs a detection result to thecontrol unit3. The sensor mounted on the elevation angle rotation sub-mechanism detects the rotation angle of the axis thereof as the angle of elevation of measurement light and outputs a detection result to thecontrol unit3. The point where the two rotation axes of the rotation mechanism intersect with each other is taken as a reference point c. Thecontrol unit3 measures the distance from the reference point c to theretro reflector11.
• Thecontrol unit3 includes adistance calculation unit31 and a changingmechanism control unit5. Thedistance calculation unit31 calculates the distance from the reference point c to the retro reflector11 (movable body2) by means of the received-light signal outputted from the firstlight reception unit413 of the distance measurementoptical system41.
• The changingmechanism control unit5 causes the changingmechanism121 to keep track of theretro reflector11. The changingmechanism control unit5 includes atracking control unit51, ajudgment unit52, apattern generation unit53, a patternemission control unit54, a restartingcontrol unit55, and amemory unit56. Thememory unit56 stores various values that are required when the changingmechanism control unit5 controls the changingmechanism121.
• Thetracking control unit51 controls the changingmechanism121 on the basis of the received-light signal outputted from the secondlight reception unit422. The changingmechanism121 is controlled in such a manner that the shift amount of return light should fall within a predetermined range. With such emission-direction control, thetracking control unit51 causes the changingmechanism121 to keep track of theretro reflector11. More specifically, as explained above, the second light reception unit (e.g., quadrisected PD)422 outputs, to thecontrol unit3, four received-light signals the level of each of which depends on the amount of return light that enters the corresponding one of four sections of a light reception plane. Thetracking control unit51 drives the changingmechanism121 in such a way as to equalize the level of the received-light signals corresponding to the upper sections of the light reception plane with the level of the received-light signals corresponding to the lower sections of the light reception plane, thereby changing the angle of elevation of measurement light. In addition, thetracking control unit51 drives the changingmechanism121 in such a way as to equalize the level of the received-light signals corresponding to the left sections of the light reception plane with the level of the received-light signals corresponding to the right sections of the light reception plane, thereby changing the angle of direction of measurement light. With the above emission-direction control, it is ensured that measurement light is always directed toward the center of theretro reflector11.
• Thejudgment unit52 includes afirst judgment unit521 and asecond judgment unit522. The function of thefirst judgment unit521 is explained below. The function of thesecond judgment unit522 will be explained later. Thefirst judgment unit521 makes a judgment on the basis of a received-light signal outputted from each of the firstlight reception unit413 and the secondlight reception unit422. For example, thefirst judgment unit521 judges whether either one or both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 is/are not greater than a predetermined first threshold value(s) or not. The predetermined first threshold values are set respectively for the firstlight reception unit413 and the secondlight reception unit422. That is, thefirst judgment unit521 judges whether or not the levels (which indicate received-light amount) of the received-light signals outputted respectively from the firstlight reception unit413 and the secondlight reception unit422 are not greater than the predetermined levels (i.e., thresholds), which are set respectively for the firstlight reception unit413 and the secondlight reception unit422.
• Thepattern generation unit53 generates a spiral pattern as a locus along which measurement light is to be emitted in a case where it has been judged by thefirst judgment unit521 that either one or both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 is/are not greater than the predetermined first threshold value(s). Theinterferometer1 searches for theretro reflector11 while emitting measurement light along the generated pattern.
• FIG. 4 is a diagram that schematically illustrates an example of a point P that is set by thepattern generation unit53 on a virtual line that goes in the direction of emission of measurement light and an X-Y plane that passes through the point P and is orthogonal to the emission direction of the measurement light according to an exemplary embodiment of the invention. Specifically, thepattern generation unit53 sets the point P in a case where it has been judged by thefirst judgment unit521 that at least one of the amounts of light received respectively at the firstlight reception unit413 and the secondlight reception unit422 is not greater than the predetermined first threshold value. As illustrated inFIG. 4, the point P lies on a virtual line that goes in the direction of emission of measurement light at the time of such a judgment (as shown by an angle of direction φ and an angle of elevation φ). In addition, the point P is set at a position on the emission-direction line away from the reference point c by a distance R. The distance R is a distance to theretro reflector11 that was calculated by thedistance calculation unit31 at the time of the judgment (to be exact, immediately before the judgment).
• Next, thepattern generation unit53 sets the X-Y plane. The X axis of the X-Y plane passes through the point P and is parallel to one of the two rotation axes of the changingmechanism121, which are orthogonal to each other, specifically, a rotation shaft b of the elevation angle rotation sub-mechanism, which changes the angle of elevation φ of measurement light. The Y axis of the X-Y plane passes through the point P and is parallel to the other of the two rotation axes of the changingmechanism121, that is, a rotation shaft a of the azimuthal angle rotation sub-mechanism, which changes the angle of direction φ of measurement light.
• FIG. 5 is a diagram that schematically illustrates an example of a spiral pattern that is generated on the X-Y plane by thepattern generation unit53 according to an exemplary embodiment of the invention. Next, as illustrated inFIG. 5, thepattern generation unit53 determines a spot coordinate Q (x, y) of measurement light as follows: (x(t), y(t))=(r(t)cos θ(t), r(t)sin θ(t)). That is, thepattern generation unit53 generates an Archimedean spiral pattern as a locus along which measurement light is to be emitted. In the present embodiment of the invention, the spiral pattern is expressed as the locus of the point Q, includes a plurality of curved successive turnings, and has a constant separation distance f between the inner turning and the outer turning of each two adjacent turnings. In the above formula and in the drawing, r(t) denotes a distance from the point P to the point Q, whereas θ(t) denotes an angle that is formed by a line segment PQ and the X axis. Note that both of r(t) and θ(t) are functions increasing monotonically over time.
• The width d of a light-receivable area, which is an area where light can be received, of each of the firstlight reception unit413 and the secondlight reception unit422 is small. For example, the width d of the light-receivable area is approximately 2.5 mm. For this reason, r(t) is set as shown in the following formula (1) in order to detect theretro reflector11 without a detection failure.
• $\begin{array}{cc}r\left(t\right)\le \frac{\theta \left(t\right)}{2\pi }·d& \left(1\right)\end{array}$
• In addition, to detect theretro reflector11 successfully, the constant separation distance f between the inner turning and the outer turning of each two adjacent turnings of the spiral is set at a value that is not larger than the width d of the light-receivable area of each of the firstlight reception unit413 and the secondlight reception unit422. In a case where the width of the light-receivable area of the firstlight reception unit413 is not the same as that of the secondlight reception unit422, the constant separation distance f between the inner turning and the outer turning of each two adjacent turnings of the spiral is set at a value that is not larger than the smaller one of the two different widths.
• Referring back toFIG. 1, the patternemission control unit54 controls the changingmechanism121 such that measurement light should be emitted along the spiral pattern generated by thepattern generation unit53. Thesecond judgment unit522 of thejudgment unit52 makes a judgment during a time period in which the patternemission control unit54 controls the changingmechanism121 for the emission of measurement light along the spiral pattern. For example, thesecond judgment unit522 judges whether both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 are greater than (or not less than) predetermined second threshold values or not. The predetermined second threshold values are set respectively for the firstlight reception unit413 and the secondlight reception unit422. In the present embodiment of the invention, it is assumed that the second threshold value is preset at a value that is equal to the first threshold value, which is used when thefirst judgment unit521 performs judgment processing. However, the scope of the invention is not limited thereto. The second threshold value may be preset at a value that is larger than the first threshold value.
• Thememory unit56 memorizes the emission direction of measurement light at the time of the judgment in a case where it has been judged by thesecond judgment unit522 that both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 are greater than the predetermined second threshold values. Then, in a case where it was judged by thesecond judgment unit522 that both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 are greater than the predetermined second threshold values, the restartingcontrol unit55 reads out and acquires information on the emission direction of measurement light at the time of the judgment from thememory unit56. The restartingcontrol unit55 controls the changingmechanism121 such that measurement light should be emitted in the acquired emission direction.
• A method for measuring a distance to themovable body2 by means of theinterferometer1 is briefly explained below.FIG. 6 is a flowchart that schematically illustrates an example of the flow of a measurement method according to an exemplary embodiment of the invention. As a first step, theinterferometer1 emits a beam of measurement light toward theretro reflector11 attached to the movable body2 (hereinafter referred to as emission step S1). The emission is triggered by, for example, an operation command given by an operator. After the emission step S1, the firstlight reception unit413 and the secondlight reception unit422 receive a beam of interference light and a beam of return light, which propagates back from theretro reflector11, respectively (hereinafter referred to as light reception step S2).
• After the light reception step S2, thefirst judgment unit521 makes a judgment on the basis of a received-light signal outputted from each of the firstlight reception unit413 and the secondlight reception unit422; for example, thefirst judgment unit521 judges whether either one or both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 is/are not greater than the predetermined first threshold value(s), where the predetermined first threshold values are set respectively for the firstlight reception unit413 and the second light reception unit422 (hereinafter referred to as judgment step S3).
• In a case where it has been judged by thefirst judgment unit521 that neither of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 is not greater than the predetermined first threshold value (S3: NO), thedistance calculation unit31 calculates the distance from the reference point c to the retro reflector11 (movable body2) on the basis of the received-light signal outputted from the first light reception unit413 (hereinafter referred to as distance calculation step S4).
• After the distance calculation step S4, thetracking control unit51 controls the changingmechanism121 on the basis of the received-light signal outputted from the secondlight reception unit422 such that the shift amount of return light should fall within a predetermined range, thereby causing the changingmechanism121 to keep track of the retro reflector11 (hereinafter referred to as tracking control step S5). After the tracking control step S5, the process returns to the step S2. Then, the steps S2 to S5 are repeated for the tracking of theretro reflector11 and the measurement of a distance to theretro reflector11.
• In a case where it has been judged by thefirst judgment unit521 that either one or both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 is/are not greater than the predetermined first threshold value(s) (S3: YES), which indicates that theinterferometer1 has now lost sight of theretro reflector11, thepattern generation unit53 generates a spiral pattern as a locus along which measurement light is to be emitted (hereinafter referred to as pattern generation step S6).
• After the pattern generation step S6, the patternemission control unit54 controls the changingmechanism121 in such a manner that measurement light should be emitted along the spiral pattern generated by the pattern generation unit53 (hereinafter referred to as pattern emission control step S7). Steps S8 and S9 explained below are executed after the pattern emission control step S7. The pattern emission control step S7 is repeated until a predetermined judgment is made in the step S9. Accordingly, the patternemission control unit54 continues controlling the changingmechanism121 till the judgment.
• Specifically, the firstlight reception unit413 and the secondlight reception unit422 receive a beam of interference light and a beam of return light respectively during a time period in which the patternemission control unit54 controls the changing mechanism121 (hereinafter referred to as light reception step S8). After the light reception step S8, thesecond judgment unit522 makes a judgment on the basis of a received-light signal outputted from each of the firstlight reception unit413 and the secondlight reception unit422; for example, thesecond judgment unit522 judges whether both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 are greater than (or not less than) the predetermined second threshold values or not, where the predetermined second threshold values are set respectively for the firstlight reception unit413 and the second light reception unit422 (hereinafter referred to as judgment step S9).
• In a case where it has been judged by thesecond judgment unit522 that either one or both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 is/are not greater than the predetermined second threshold value(s) (S9: NO), the process returns to the pattern emission control step S7. In this case, the steps S7, S8, and S9 are repeated for the emission of measurement light along the spiral pattern.
• In a case where it has been judged by thesecond judgment unit522 that both of the amount of light received at the firstlight reception unit413 and the amount of light received at the secondlight reception unit422 are greater than the predetermined second threshold values during a time period in which the patternemission control unit54 controls the changingmechanism121 for the emission of measurement light along the spiral pattern (S9: YES), which indicates that theinterferometer1 has now found theretro reflector11, thememory unit56 memorizes the emission direction of measurement light at the time of detection (hereinafter referred to as memory step S10). After the memory step S10, the restartingcontrol unit55 acquires information on the emission direction of measurement light from thememory unit56 and controls the changingmechanism121 such that measurement light should be emitted in the acquired emission direction (hereinafter referred to as restarting control step S11).
• As explained above, in the present embodiment of the invention, the restartingcontrol unit55 controls the changingmechanism121 such that measurement light should be emitted in the emission direction at the time of detection in a case where theretro reflector11 has now been detected. If the measurement of a distance to theretro reflector11 were resumed immediately upon the detection of theretro reflector11, the emission direction of measurement light could be shifted from the direction in which theretro reflector11 has now been found due to the inertial force of the changingmechanism121 that acts during a time period from the detection to the restart of measurement. Therefore, there is a risk of failing to detect theretro reflector11. The reason why the restarting control processing explained above is performed is to avoid such a risk. After the restarting control step S11, the process returns to the step S2. Then, the procedure for the tracking of theretro reflector11 and the measurement of a distance to theretro reflector11 is resumed.
• The present embodiment of the invention explained above produces the following advantageous effects.
• (1) Upon losing sight of theretro reflector11, theinterferometer1 emits a beam of measurement light along a predetermined pattern so as to search for theretro reflector11. Theinterferometer1 restarts the tracking of theretro reflector11 when theretro reflector11 has now been found. Therefore, it is possible to resume the measurement of a distance to theretro reflector11 even when theinterferometer1 loses sight of theretro reflector11.
• (2) The emission pattern of measurement light that is used when theinterferometer1 searches for theretro reflector11 is a spiral pattern whose center is a given point P that lies in the direction of emission of measurement light when theinterferometer1 lost sight of theretro reflector11. With such a feature, it is possible to search for theretro reflector11 efficiently without a detection failure, or with a substantially reduced risk of a detection failure.
• (3) The emission pattern of measurement light that is used when theinterferometer1 searches for theretro reflector11 is a spiral pattern that includes a plurality of curved successive turnings and has the constant separation distance f between the inner turning and the outer turning of each two adjacent turnings. In addition, the constant separation distance f between the inner turning and the outer turning of each two adjacent turnings of the spiral is set at a value that is not larger than the width d of the light-receivable area of each of the firstlight reception unit413 and the secondlight reception unit422. With such a feature, it is possible to substantially reduce the risk of a detection failure.
• (4) If the measurement of a distance to theretro reflector11 were resumed immediately upon the detection of theretro reflector11 as a result of search for theretro reflector11, the emission direction of measurement light could be shifted from the direction in which theretro reflector11 has now been found due to the inertial force of the changingmechanism121 that acts during a time period from the detection to the restart of measurement. Therefore, there is a risk of failing to detect theretro reflector11. In contrast, in the present embodiment of the invention, when theretro reflector11 has now been found as a result of search for theretro reflector11, emission direction control processing is performed to direct measurement light in the direction in which theretro reflector11 has been found. Thereafter, the measurement of a distance to theretro reflector11 is resumed. With such a feature, it is possible to detect theretro reflector11 without fault. Accordingly, the measurement of the distance to theretro reflector11 can be resumed reliably.
Variation Examples of Foregoing Embodiment
• The scope of the invention is not limited to the foregoing embodiment. Various modifications, improvements, and the like that are made within a range in which an object of the invention is achieved are encompassed therein.FIG. 7 is a diagram that schematically illustrates a spiral pattern according to a variation example of the foregoing embodiment of the invention. In the foregoing embodiment of the invention, it is explained that thepattern generation unit53 generates a “curved spiral pattern” (i.e., ordinary spiral pattern) that includes a plurality of curved successive turnings and has the constant separation distance f between the inner turning and the outer turning of each two adjacent turnings. However, the scope of the invention is not limited to such an example. As illustrated inFIG. 7, thepattern generation unit53 may generate a “non-curved spiral pattern” (e.g., square spiral pattern) that includes a plurality of non-curved successive turnings and has a constant separation distance f between the inner turning and the outer turning of each two adjacent turnings. A plurality of line segments constitutes the plurality of non-curved successive turnings. The term “spiral” used in the appended claims is not intended to limit the scope of the invention to various curved patterns only.
• In the foregoing embodiment of the invention, it is explained that thepattern generation unit53 generates a spiral pattern. However, the scope of the invention is not limited to such an example. For example, thepattern generation unit53 may generate a sequential scanning pattern along which theinterferometer1 performs sequential scanning operation for rows (or columns) in a direction from one side to the other, for example, from top to bottom.
• In the foregoing embodiment of the invention, thepattern generation unit53 sets the center point P of a spiral pattern. It is explained that the center point P lies in the direction of emission of measurement light when theinterferometer1 lost sight of theretro reflector11. In addition, the center point P is set at a position away from the reference point c by a certain distance to theretro reflector11, which was calculated by thedistance calculation unit31 when theinterferometer1 lost sight of theretro reflector11. Notwithstanding the foregoing, however, thepattern generation unit53 may set the center point P of a spiral pattern at any arbitrary position in the direction of emission of measurement light when theinterferometer1 lost sight of theretro reflector11.
• In the foregoing embodiment of the invention, it is explained that the changingmechanism control unit5 judges whether theinterferometer1 has now lost sight of theretro reflector11 or not on the basis of a received-light signal outputted from each of the firstlight reception unit413 and the secondlight reception unit422 and judges whether theinterferometer1 has now found theretro reflector11 or not on the basis thereof to execute the pattern emission control step S7 and the tracking control step S5. However, the scope of the invention is not limited to the foregoing example. The changingmechanism control unit5 may perform each judgment processing on the basis of either one of received-light signals outputted respectively from the firstlight reception unit413 and the secondlight reception unit422 only to execute the pattern emission control step S7 and the tracking control step S5.
• That is, on the basis of either one of received-light signals outputted respectively from the firstlight reception unit413 and the secondlight reception unit422 only, thefirst judgment unit521 may judge whether the amount of light received at the one light reception unit mentioned above (413 or422) is not greater than the predetermined first threshold value. Then, thepattern generation unit53 may generate a spiral pattern in a case where it has been judged by thefirst judgment unit521 that the amount of light received at the one light reception unit is not greater than the predetermined first threshold value. In like manner, on the basis of the one of received-light signals outputted respectively from the firstlight reception unit413 and the secondlight reception unit422 only, thesecond judgment unit522 may judge whether the amount of light received at the one light reception unit is greater than the predetermined second threshold value. The steps S10, S11, and S2 to S5 are executed in a case where it has been judged by thesecond judgment unit522 that the amount of light received at the one light reception unit is greater than the predetermined second threshold value. Thetracking control unit51 causes the changingmechanism121 to keep track of theretro reflector11 in the step S5. The above modified configuration in which the changingmechanism control unit5 performs each judgment processing on the basis of either one of received-light signals outputted respectively from the firstlight reception unit413 and the secondlight reception unit422 only to execute the pattern emission control step S7 and the tracking control step S5 produces the same advantageous effects as those of the foregoing embodiment, or similar thereto. Besides the foregoing advantages, the modified configuration has an advantage of simplicity because each judgment processing is performed on the basis of either one of received-light signals outputted respectively from the first and second light reception units only.
• In the foregoing embodiment of the invention, theinterferometer1 starts a search for theretro reflector11 upon losing sight of theretro reflector11. Theinterferometer1 may search for theretro reflector11 immediately after activation. For example, the search upon activation can be carried out as follows. Theinterferometer1 emits a beam of measurement light in an arbitrary direction immediately after activation. In addition, theinterferometer1 determines the center point P of a spiral pattern at an arbitrary position in the emission direction of the beam of measurement light. Next, theinterferometer1 sets a plane that passes through the point P and is orthogonal to the emission direction of the measurement light. Then, theinterferometer1 generates a spiral pattern whose center is the point P on the plane. Theinterferometer1 searches for theretro reflector11 while emitting measurement light along the generated pattern.
• The invention can be applied to a tracking-type laser interferometer.

Claims (6)

1. A tracking-type laser interferometer comprising:

a light source;

a retro reflector that is attached to a movement member and reflects light propagating from the light source;

a reference plane that reflects light coming from the light source;

a first light receiving section that receives interference light turned from return light and reference light, the return light propagating back from the retro reflector, the reference light being reflected at the reference plane, the first light receiving section outputting a received-light signal dependent on received-light amount and a change in a distance to the retro reflector upon receiving the interference light;

a second light receiving section that receives the return light to output a received-light signal dependent on received-light amount and shift amount of the return light;

a changing mechanism that changes a direction of emission of the light propagating from the light source;

a changing mechanism controlling section that controls the changing mechanism based upon the received-light signal outputted from the second light receiving section such that the shift amount is within a predetermined range to cause the changing mechanism to keep track of the retro reflector, the changing mechanism controlling section including

a first judging section,

a second judging section,

a pattern emission controlling section, and

a tracking controlling section; and

a distance calculating section that calculates the distance from a predetermined reference point to the retro reflector by means of the received-light signal outputted from the first light receiving section,

wherein the first judging section judges whether at least one of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section is not greater than a predetermined first threshold value, which is set individually for each of the first and second light receiving sections,

wherein the pattern emission controlling section controls the changing mechanism such that the light propagating from the light source should be emitted along a predetermined pattern in a case where it has been judged by the first judging section that at least one of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section is not greater than the predetermined first threshold value,

wherein the second judging section judges whether both of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section are greater than, or not less than, predetermined second threshold values, which are set respectively for the first and second light receiving sections, and

wherein the tracking controlling section controls the changing mechanism such that the shift amount falls within the predetermined range to cause the changing mechanism to keep track of the retro reflector in a case where it has been judged by the second judging section that both of the received-light amount at the first light receiving section and the received-light amount at the second light receiving section are greater than, or not less than, the predetermined second threshold values.

2. A tracking-type laser interferometer comprising:

a light source;

a retro reflector that is attached to a movement member and reflects light propagating from the light source;

a reference plane that reflects light coming from the light source;

a first light receiving section that receives interference light turned from return light and reference light, the return light propagating back from the retro reflector, the reference light being reflected at the reference plane, the first light receiving section outputting a received-light signal dependent on received-light amount and a change in a distance to the retro reflector upon receiving the interference light;

a second light receiving section that receives the return light to output a received-light signal dependent on received-light amount and shift amount of the return light;

a changing mechanism that changes a direction of emission of the light propagating from the light source;

a changing mechanism controlling section that controls the changing mechanism based upon the received-light signal outputted from the second light receiving section such that the shift amount falls within a predetermined range to cause the changing mechanism to keep track of the retro reflector, the changing mechanism controlling section including

a first judging section,

a second judging section,

a pattern emission controlling section, and

a tracking controlling section; and

a distance calculating section that calculates a distance from a predetermined reference point to the retro reflector by means of the received-light signal outputted from the first light receiving section,

wherein the first judging section judges, on the basis of either one of the received-light signals outputted respectively from the first and second light receiving sections only, whether the received-light amount at said one light receiving section is not greater than the predetermined first threshold value,

wherein the pattern emission controlling section controls the changing mechanism such that the light propagating from the light source should be emitted along a predetermined pattern in a case where it has been judged by the first judging section that the received-light amount at said one light receiving section is not greater than the predetermined first threshold value,

wherein the second judging section judges, based upon said one of the received-light signals outputted respectively from the first and second light receiving sections only, whether the received-light amount at said one light receiving section is greater than, or not less than, the predetermined second threshold value, and

wherein the tracking controlling section controls the changing mechanism such that the shift amount falls within the predetermined range to cause the changing mechanism to keep track of the retro reflector in a case where it has been judged by the second judging section that the received-light amount at said one light receiving section is greater than, or not less than, the predetermined second threshold value.

3. The tracking-type laser interferometer according toclaim 1, wherein the predetermined pattern is a spiral pattern that starts at a given point and goes away from the given point outward while turning around the given point as the center of the spiral pattern on a plane; the given point lies in a direction of emission of light propagating from the light source when it is judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value; the plane passes through the given point; and the plane is orthogonal to the direction of emission of the light propagating from the light source.

4. The tracking-type laser interferometer according toclaim 3, wherein the given point is set at a position away from the reference point by a certain distance to the retro reflector calculated by the distance calculating section when judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value; the predetermined pattern is a spiral pattern that includes a plurality of curved successive turnings and has a constant separation distance between the inner turning and the outer turning of each two adjacent turnings; and the constant separation distance between the inner turning and the outer turning of each two adjacent turnings of the spiral is set at a value that is not larger than the width of a light-receivable area of the light receiving section for which it has been judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value.

5. The tracking-type laser interferometer according toclaim 2, wherein the predetermined pattern is a spiral pattern that starts at a given point and goes away from the given point outward while turning around the given point as the center of the spiral pattern on a plane; the given point lies in a direction of emission of light propagating from the light source when it is judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value; the plane passes through the given point; and the plane is orthogonal to the direction of emission of the light propagating from the light source.

6. The tracking-type laser interferometer according toclaim 5, wherein the given point is set at a position away from the reference point by a certain distance to the retro reflector calculated by the distance calculating section when judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value; the predetermined pattern is a spiral pattern that includes a plurality of curved successive turnings and has a constant separation distance between the inner turning and the outer turning of each two adjacent turnings; and the constant separation distance between the inner turning and the outer turning of each two adjacent turnings of the spiral is set at a value that is not larger than the width of a light-receivable area of the light receiving section for which it has been judged by the first judging section that the received-light amount is not greater than the predetermined first threshold value.

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